(a) Field of the Invention
The invention relates to a collimator and, in particular, to a fiber collimator employing a graded-index multi-mode fiber to collimate light rays.
(b) Description of the Related Art
In optical devices or systems, a collimator is a commonly used optical device, which collimates an optical signal into a parallel light so as to reduce divergence of light beam.
In conventional technology, the collimator employs a graded-index lens (GRIN lens) to change the divergent angle of divergent light entered from a single-mode fiber such that the divergent light is changed into parallel emitted light to be coupled to other optical devices.
However, the volume of such a collimator is relatively large and the outer diameter of the collimator is about 1.8 mm. Thus, it is not suitable to be used in optical devices with a small size, for instance, a micro-electromechanical system (MEMS) switch or a planar waveguide device.
In view of the above, an object of the invention is to provide an optical fiber collimator and a method for fabricating the same, wherein the collimator occupies a smaller space, which suits optical devices having a smaller volume.
To achieve the above object, the optical fiber collimator of the invention includes a single-mode fiber and a graded-index multi-mode fiber. The graded-index multi-mode fiber is spliced with the single-mode fiber and has a predetermined length so as to collimate light rays. The waveform of the light rays within the graded-index multi-mode fiber has a period P during transmission and the length of the graded-index multi-mode is substantially 0.25xc3x97Nxc3x97P, wherein N is a positive odd number.
The graded-index multi-mode fiber is spliced with single-mode fiber by fusion splicing or adhesive splicing. A protective film covers the coupling point between the single-mode fiber and the graded-index multi-mode fiber so as to prevent the single-mode fiber and the graded-index multi-mode fiber from breakage at the coupling point.
After the single-mode fiber is spliced with the graded-index multi-mode fiber, the length of the graded-index multi-mode fiber is adjusted by heating or stretching, such that the substantial length of the graded-index multi-mode fiber is 0.25xc3x97Nxc3x97P. In addition, a UV-sensitive graded-index multi-mode fiber is used to change the gradient of the refractive index, and in turn, the period P of the light rays transmitted in the graded-index multi-mode fiber is changed.
In the course of adjustment of the graded-index multi-mode fiber, a light beam is coupled to the graded-index multi-mode fiber, and a power meter is used to detect the output of the single-mode fiber so as to determine whether the graded-index multi-mode fiber has been fully adjusted. In addition, the light beam is also coupled to the single-mode fiber and a beam scanner is used to inspect the size of the section of the beam output from the graded-index multi-mode fiber so as to determine whether the graded-index multi-mode fiber has been fully adjusted.